I discuss issues pertaining to the practice of neuropathology -- including nervous system tumors, neuroanatomy, neurodegenerative disease, muscle and nerve disorders, ophthalmologic pathology, neuro trivia, neuropathology gossip, job listings and anything else that might be of interest to a blue-collar neuropathologist.

Thursday, December 27, 2007

Medial lemniscus

Brain cutting sessions with residents can get repetitive after a while unless you have new neuroanatomy information to share with them every so often. To that end, here's a nice description of the course of the medial lemniscus as it ascends through the brainstem written with a visual metaphor that will really stick in your mind. You can use this metaphor in describing the medial lemniscus during your next brain cutting session. The source is my favorite basic neuropathology book: "Practical Review of Neuropathology" by Fuller and Goodman. From page 113:

"The ascending proprioceptive and fine touch system has its relay nuclei, the nucleus gracilus for lower extremity and trunk, and the nucleus cuneatus for upper extremity, neck, and upper trunk...at the cervicomedullary junction. The second order neurons from these nuclei decussate to form the medial lemniscus that ascends through the brainstem to eventually innervate the ventral posterior lateral (VPL) nuclues of the thalamus. In the medulla, the medial lemniscus is arranged as a vertical (ventral to dorsal ) strip of fibers in the midline with the upper extremity fibers being most dorsal and the lower extremetiy fibers being most ventral; i.e., the sensory homunculus in the medulla is 'standing straight up'. The medial lemniscus rotates to the horizontal in the pons and by the time it reaches the midbrain has rotated further so that the fibers from the lower extremity are now more dorsal than those of the upper extremity. Imagine a reveler on Bourbon Street in New Orleans. Early in the evening, the individual is standing vertically against a lamppost enjoying the libations and this is analogous to the position of the medial lemniscus in the medulla. By mid-evening, the partygoer slides down the lamppost and lays on the street analogous to the increasingly horizonatal disposition of the medial lemniscus in the pons. Finally, the besotted reveler is pulled from the street by his ankles by the local constabulary analogous to the 'feet up' orientation of the medial lemniscus in the midbrain."

I should note that I had to correct a few typos in this as I transcribed it. For example, the authors write "Bourdon Street" instead of "Bourbon Street" in New Orleans. And they refer to the "nucleu gracilus" instead of the "nucleus gracilus". That being said, I still think that Fuller and Goodman provide a wonderful analogy that really helps in explaining the anatomy of the medial lemniscus!

I hear the analogy, but have a problem with it.The analogy implies that as the ML rotates vertical to horizontal, the ventral portion (feet) slides laterally, pulling the dorsomedial portion down to become ventromedial (as if the homunculus is being dragged by its feet, and the neck follows). That's not the case. In fact, the ML realignment is a point rotation at the dorsal end (homunculus pivots on its neck), and the ventromedial end swings up to become dorsolateral - imagine the drunk's head getting stuck to the lamppost (perhaps he licked it when it was cold), and his feet swing up, so that he is levitating or singing in the rain.